An understanding of the molecular basis of action, or mechanism, of proteolytic enzymes is a long-term objective of biochemical investigation, realization of which would have multiple benefits with regard to medical problems associated with protein metabolism. For metalloproteases, it is proposed to develop a new class of transition state-analog inhibitors based upon the phenol and upon the sulfone imine functional groups, for which preliminary evidence has indicated unusual efficacy. These promise to provide unique insight into the peptide cleavage mechanism. Also to be investigated are proline-specific proteases, which represent a previously underdeveloped topic. New designs of affinity labels and mechanism-based inactivators have been devised. A specific feature of the latter effort is broad potential applicability of materials so developed to a range of current developments in protein chemistry.